Method of protecting metal surfaces



Patented Oct. 13, 193i UNITED STATES PATENT OFFIC/E SUMNER REDWAY MASON, OF WILMETTE, ILLINOIS, ASSIGNOR 1E0 WESTERN ELECTRIC COMPANY, INCORPOBATED, OF NEW YORK, N. Y., CORPORATION OF NEW YORK METHOD OF IPROTECTINQ -M ETAL SURFACES No Drawing. 1

- economical method of rendering metal surfaces capable of resisting the action of moisture or other influences tending to corrode such surfaces. This invention is related to that disclosed in my copending application Serial No. 227,061, filed October 18, 1927.

In accordance with the general features of the invention an article having a metal surface which it is desired to protect, is connected as a cathode in an electrolytic cell in which the bath employed contains a compound of a metallic element and an initial electric current of sufliciently low cathode current density to insure substantially no deposition of metal is passed through thecell for a short period of time, after which the article is removed from the cell and dried.

It has been found by experiment that when such metals as iron, .zinc, cadmium, copper, silver, etc. are connected as cathodes in an electrolytic cell in which the bath contains a material, such as chromic acid, compr smg a compound of a metallic element in which the element forms a part of an acid radical and when a current of sufliciently low cathode current density to provide conditions under which no metal is deposited, ispassed through the cell for a short period of time the surfaces of the articles are rendered passive to the extent that they will ,resist the corrosive action of atmospheric agents for a very'long period of time. The surfaces rendered passive are also quite resistant to the action of sulphur, acid-s, such as nitric acid and hydrogen sulphide, and numerous other corrosive substances even after the treated articles have been subjected to abrasion. This phenomenon provides a very effective method for protecting metal surfaces and it constitutes the subject matter of the present invention.

Although it. is to be understood that the method'of protectingmetal surfaces forming the "subject matter of this invention is applicable to numerous metals and that 'the ingredients and conditions under which the method is practiced may be varied within a wide range, a specific embodiment of the ineffective and Application filed October 18, 1927. Serial No. 227,060. 1

'vention will'be described in detail in order to enable persons skilled in the art to practice the invention. 7

One embodiment of the invention consists in a method for protecting the surfaces of articles made of iron. In this embodiment of the invention an electrolytic cell is emvloyed'in which the bath comprises a water solution containing 22 grams per liter of chromic. acid ((310,) in which is connected an anode of any suitable insoluble material such as iron, platinum or lead. The article to be treated is connected as a cathode in the above described electrolytic cell, an electric current of a cathode current density of 25 to amperes per square foot is passed through the cell for a short period of time, and the temperature of the bath is maintained at room temperature (2025 C.) during the passage of the current. For rendering the surfaces resistant to the action of atmospheric agents three minutes has proven to be a satisfactory length of time for the electrolytic treatment. After this treatment the article is removed from the electrolytic cell and dried in any suitable manner. An iron article so treated will be found to have exceptional resistance to atmospheric corrosion and.

will not rust to any great extent even if scratched unless exposed to extreme conditions for a very long period of time.

It is believed that the embodiment of the invention just described will sufficiently illustrate how the invention may be practiced, but it is, of course, to be understood that the invention is not limited to the'specific exam-' ple given hereinbefore. The invention may be practiced with satisfactory. results on tity to almost saturation but for the sa obtained with baths containing chromic acid in amounts varying from a very small uanteconomy the bath should preferably contain less than 50 grams per liter of chronnc acld. Nor does the bath need to contain only a chromic acid in'water solution, .but add1-.

tional conducting materials may present in order to vary the operating conditions of the bath.

It is also possible to. obtain results a? to those obtained by the use of chromic acid by employing other compounds containing a metal as' a' part of an acid radical or an amphoteric metallic element and satlsfactory results have beenbbtainedby employing such materials as sodium, potassium, ammonium and lithium stannates, tungstates, chromates, dichromates, molybdates, etc. Some compounds, suchas manganese sul hate, containing a metallic element whic does not form. part of an acid radical, also give results similar to those obtained by the use of the above'named compounds.

Also "the amount of. current flowing through the cell may be varied over a wide range so long as the initial cathode current density is maintained below the range within which a metallic material would be deposited from the particular bath used in quantities more than traces. The tempera ture of the bath may also be varied to provide-the optimum conditions of operation obtainable with the particular bath and current density employed, and also the time of treatment may be varied. Satisfactory'results may be usually obtained, however, when the bath is operated at room temperature (20 to 25 C.) and the article is treated for from three to fifteen minutes in the electrolytic cell.

A satisfactory theory of the reactions which take place during the above described operations has not yet been developed, but some or all of a number of changes might possibly occur. For example, the surface of the metal which it is desired to protect may be oxidized or otherwise chemically changed. Or a physical change or displacement may be efiected in themolecules comprising the surface layers of the metal so that the surface of the 'metalis rendered passive. Or it may be possible that the metal surface is coated with an extremely thin film composed of a compound or compounds derived from the electrolytic bath. Whatever the changes or reactions which occur may be, a treatment embodying the invention renders the surface of the treated metal resistant to corrosion, and at the same time the weight and dimensions of the treated article remain substantially unchanged. Also the appearance of the articles remains practically unchanged in most instances after treatment. For example, ironarticles originally having bright,

Ttion of metallic aeaaeaa clean surfaces usuallg exhibit a similar ap earance with possib Z a sli ht iridescenc'e ei-ng noticeable after aving een treated in a chromic acid bath such as has been described hereinbefore, but thereis no evidence that the articles are coated with any colored or colloidal material of the t es sometimes found in electroplating baths or the deposichromium.

The method described above provides an extremely simple and inexpensive means whereby surfacesof metal articles may be protected against corrosion and abrasion without any material variation in the weight or dimensions of the article. The treatment. is especially eifective in treating irregular, shaped articles for not only the plane 'surfaces of the articlesare protected, but the protective effects are also obtained in corners, crevices, and the walls of holes in the articles.

By the statement employed hereinbefore V in the specification and also in the appended claims, that the cathode current density is maintained at a value below those in the range within which a metal is deposited is meant a cathode current density of such value that no more than traces of a metallic material are deposited. The term amphoteric metallic element 'as used herein in the specification and claims is intended to designate those metallic elements which may form one group of compounds in which the element comprises part of an acid radical and may form another group of compounds in which it acts as a base forming element. Thus, chromium may form compounds of the nature of sodium chromate (Na2GI'O4) in which the chromium forms part of an acid radical, or may form compounds such as chromic chloride (Cr C1 in which it acts as a base forming element,

What is claimed is:

l. Aprocess for protecting surfaces of metal articles, which consists in connectin an article as a cathode in an electrolytic cell including a bath containing chromic acidas an electrolyte, passing an electric current of a cathode current density below the rangb in which chromium is deposited through the cell for from three to fifteen minutes, and maintaining the temperature of the bath at room temperature.

2. A process for protecting surfaces of metal articles, which consists in connectin an article as a cathode in an electrolytic cell including a bath containing less than 50 grams per liter of chromic acid as an electrolyte, and passing an electric current of a cathode current density below the range in which chromium .is deposited through the cell.

3. A process for-protecting surfaces ofmetal articles, which consists in conne including a bath c'ont'ainin 22 liter of ams per i chromic acid as an e ectro? passing an electric current of a cath e current density is deposited throu to fifteen minutes, and maintainingthe tembelow the range in which chromium gh the cell for from three perature of the bath at room temperature.

Inw

itness whereof, I hereunto subscribe my name this'30th day of September A. D.,

, SUMNER REDWAY MASON. 

